1. Field of the Invention
The present invention relates to a motor, and in particular, to a motor having a hydrodynamic bearing structure which makes use of a lubricating fluid between adjacent surfaces of a rotary member and a stationary member, the motor being adapted to rotate a data storage media such as a hard disk in a hard disk drive.
2. Background Information
Conventionally, there has been known and used a motor equipped with a hydrodynamic bearing device which uses fluid pressure created between a shaft body and a sleeve structure in order to rotatably support the shaft body and the sleeve structure such that one of them is rotatable relative to the other. An example of such a motor is described in detail below with reference to FIG. 1.
FIG. 1 is a longitudinal sectional view schematically showing structure of a prior art motor 150 equipped with a bearing device using dynamic pressure of a fluid lubricant. As illustrated in FIG. 1, the conventional motor 150 equipped with a bearing configuration using hydrodynamic pressure has a cylindrical shaft housing 151 for rotatably supporting a rotary shaft 154, and the cylindrical shaft housing 151 has a large diameter base portion 151a. The outer peripheral surface of a lower portion of the large diameter base portion 151a is fixedly fitted in a circular engaging hole 152a of a base plate 152 of a recording medium drive device. The base portion 151a is integrally formed with an annular ring-shaped plate portion 151b. The annular ring-shaped plate portion 151b is further integrally formed with a small diameter sleeve portion 151c that is coaxially aligned with the base portion 151a and located above the base portion 151a. Further, a thrust cover 153 is fixedly engaged with an inner recessed surface of the base portion 151a adjacent to the lower end thereof, thereby blocking and sealing a disk shaped internal space defined within the base portion 151a.
In this way, a shaft supporting structure is thus formed with the use of the cylindrical shaft housing 151 and the thrust cover 153. The rotary shaft 154 is supported in a vertical orientation within the sleeve portion 151c of the cylindrical shaft housing 151 by a fluid lubricant 155, such as lubricating oil, that fills a clearance gap formed between surfaces of the rotary shaft 154 and the sleeve portion 151c due to liquid capillary action. The surfaces of the rotary shaft 154 and adjacent surfaces of the sleeve portion 151c serve as upper and lower radial bearings 170 and 171 using dynamic pressure of the lubricant 155 to support the rotary shaft 154 within the sleeve portion 151c such that the rotary shaft 154 is freely and relatively rotatable within the sleeve portion 151c.
A ring-shaped thrust plate 156 is fixedly fitted to a lower end of the rotary shaft 154 and is positioned in the disk shaped internal space that is defined within the base portion 151a. A clearance gap defined in the disk shaped internal space between the surfaces of the ring-shaped thrust plate 156 and the inner surface of the base portion 151a, the inner surfaces of the annular ring-shaped plate portion 151b and an upper surface of the thrust cover 153 is filled with lubricant 155 retained therein by capillary action. Upper and lower surfaces of the ring-shaped thrust plate 156 and adjacent surfaces of the base portion 151a, the plate portion 151b and thrust cover 153 serve as upper and lower thrust bearings allowing the annular ring-shaped thrust plate 156 to rotate freely within the cylindrical shaft housing 151 in combination with the dynamic pressure of the lubricant 155. In this manner, with the use of upper and lower hydrodynamic radial bearings 170 and 171 and upper and lower hydrodynamic thrust bearings, a hydrodynamic fluid bearing structure is formed which makes use of the hydrodynamic pressure of the fluid lubricant 155 during the relative rotation between the rotary shaft 154 (with the thrust plate 156) and the cylindrical shaft housing 151.
An annular groove 157 is formed at approximately a middle portion of the rotary shaft 154 separating the upper and lower radial bearings 170 and 171. The annular groove 157 is surrounded by an adjacent portion of the inner surface of the sleeve portion 151c forming an annular air space 159 that communicates with atmosphere outside the motor via a breather hole 158 formed on the sleeve portion 151c.
Herringbone grooves 160a and 160b are formed on lower and upper surfaces, respectively, of the thrust plate 156. Herringbone grooves 160c and 160d are formed on inner surfaces of the sleeve portion 151b below and above the annular air space 159, respectively. In response to rotation of the rotary shaft 154, radial load supporting pressure and thrust load supporting pressure are generated in the lubricant 155 in and about the herringbone grooves 160a, 160b, 160c and 160d.
A stator 161 formed with coil windings (not shown) around a stator core (not shown) is fixed on an outer surface of the sleeve portion 151c. A cup-like rotor hub 162 is formed with an outmost enclosure wall 162a that encircles the stator 161. The upper end of the rotary shaft 154 extends into a center hole formed in the cup-like rotor hub 162 such that the rotary shaft 162 is engaged and fixed to the cup-like rotor hub 162. A rotor magnet 163 is secured on an internal surface of the outmost enclosure wall 162a of the rotor hub 162 such that the rotor magnet 163 radially faces the stator 161 with a predetermined clearance space maintained therebetween thereby forming a rotation driving structure.
When using the above-described conventional hydrodynamic fluid bearing assembly having upper and lower radial hydrodynamic bearings and upper and lower hydrodynamic thrust bearings, the ring-shaped thrust plate 156 is used in the hydrodynamic thrust bearing structure. In order to ensure a stabilized support for the rotary shaft 154 in the axial direction and thereby minimize possible vibrations in that direction, both upper and lower surfaces of the thrust plate 156 must be used to form upper and lower thrust bearings. However, there is a problem associated with using both upper and lower surfaces of the thrust plate as bearings in that bearing losses due to, for instance, fluid friction, may be large and as a result the electric efficiency of the motor may be low.
When a hydrodynamic bearing motor 150 described above is to be installed in a thin hard disk drive (HDD) whose thickness is, for example, less than 5 mm, the sleeve portion 151c, the rotary shaft 154, the stator 161 and the rotary magnet 163 somehow have to be made shorter in the vertical direction, as viewed in FIG. 1. When a hydrodynamic bearing motor 150 is used in such a thin and low-noise hard disk drive with the motor being provided with both upper and lower radial hydrodynamic bearings, upper and lower thrust hydrodynamic bearings provided on upper and lower sides of a thrust plate 156, the rotary shaft 154 and the thrust plate 156 may be supported stably with minimal axial vibrations. Although the thrust hydrodynamic bearings on both sides of the thrust plate may ensure stability of the rotary shaft 154 in the axial direction, the grooves 160a and 160b generate large viscous resistance against the flow of the lubricant 155 resulting in bearing loss making the motor electrically inefficient.
Also known is a hydrodynamic bearing motor in which no thrust plate 156 is employed. Instead a thrust bearing is formed on an end surface of a rotary shaft. In this case, although bearing loss is small and the motor is relatively electrically efficient, the motor requires some kind of axial movement prevention mechanism to prevent movement of the rotary shaft in the axial direction, since the rotary shaft does not have a projection such as a thrust plate for retaining the rotary shaft within a motor housing. When impact is applied to the motor, large axial movement of the rotary shaft may occur causing undesirable contact between a magnetic head and a data storage medium such as a hard disk on which the magnetic head writes and reads data, thereby adversely affecting the reading and writing functions of the magnetic head and possibly damaging the magnetic head and the recording medium.
In view of the above, there exists a need for a motor which overcomes the above mentioned problems in the prior art. The present invention attempts to solve the problems associated with the above described related art, as will become apparent to those skilled in the art from the following description of the present invention.